A relatively small part. Apple computer sales are not that impressive. They may be a large individual conntributor, but they alone do not sell enough to make a big difference.
Yes, Macs are a pretty small figure. The new iPhone modem deal Intel struck with Apple for around $1.5 billion is a bigger deal, which would be in the ballpark of 2% of Intel's ~$55 billion in revenues.
But Intel likes Apple Macs as a show pony. Intel has for years been desperately trying to convince hardware manufacturers to make both more consumer appealing PCs and new consumer devices around their chips. PC makers kept making the same old big beige towers, with the only differentiating factors being price (race to the bottom) and speed.
Apple doesn't like to compete in that space and likes to do other things and pushes on different features of chips that allow them to make smaller machines, quiet machines, fully integrated machines, battery friendly machines, and focus on industrial design.
Intel knows this and likes to use Apple to push other manufacturers to think in similar ways. The results have been pretty good as there are now PC competitors that make things like the All-in-one-flatscreen iMac, and the Macbook Air. If you remember way back before the iPhone, the original Apple TV was Intel based and showed Intel chips in mainstream consumer non-PC use cases. (This is different than concept demos because this was a real shipping product that actually managed to make enough money to sustain itself.) Also, Apple's obsession with reduction, like eliminating ports has been good for Intel because Apple helps encourage adoption of new standards they want to put out. For example, Apple embraced Thunderbolt at the beginning and was willing to drop legacy ports while most manufactures would normally hedge by keeping legacy ports. Apple pushing hard on this helps makes the 3rd party adoption faster which emboldens manufacturers to migrate faster themselves.
